This invention relates to a process for recovering metallic lead from lead by-product materials of the character of those produced in the manufacture of tetraalkyllead compounds, such as tetraethyllead.
Tetraethyllead has been manufactured commercially for many years by reacting an excess of ethyl chloride with lead-monosodium alloy. In such reaction, the sodium in the alloy is mostly converted to sodium chloride, about 25% of the lead in the alloy is converted to tetraethyllead, and most of the rest of the lead in the alloy is converted to metallic lead in finely divided form. After the reaction has been completed, the excess ethyl chloride is distilled off, and the reaction mass is then drowned in water and the tetraethyllead removed by steam distillation in the presence of a still aid which largely prevents agglomeration of the finely-divided metallic lead particles. The steam still residue comprises a suspension of the by-product lead particles in a dilute solution of sodium chloride. This suspension is conveyed to a sludge pit where it is allowed to settle, forming an upper layer of aqueous salt solution and a lower layer of wet sludge which is composed mainly of the finely-divided lead mixed with about 8% to about 20% by weight of the aqueous salt solution. The water layer is drawn off, the wet sludge is washed with water to remove most of the salt and then the sludge is dried to remove most of the water.
The resulting dried by-product lead sludge is impure, containing sodium chloride, lead chloride, sodium hydroxide and, in some cases, lead sulfate, lead sulfide and lead chromate (formed by reaction of the lead with the still aid), usually in a proportion of less than 1% by weight. In addition, the lead particles are coated with lead oxide. Usually, the lead oxide is present in a proportion of from about 2% to about 10% by weight, frequently as high as about 20% and, in extreme cases, as high as about 30%. It is possible, in some cases, to keep the amount of lead oxide down to 0.1-0.2% by weight. The by-product lead sludge will sometimes contain iron salts, such as ferric chloride, ferric sulfate and iron sulfides, when an iron compound (usually ferrous sulfate) is employed as an ingredient of the still aid. By-product lead sludge of the above character is also produced in the production of other lead alkyls by similar reactions and in the production of tetraethyllead and like lead alkyls by other reactions.
The recovery of the lead in refined form from such impure by-product lead sludge has been difficult and has required a series of treatments which are inconvenient and costly. Usually, the impure by-products lead sludge has been refined by melting it in a reverberatory furnace at a temperature of from about 700.degree. C. to about 900.degree. C. to form an upper layer of slag and a lower layer of molten lead consisting of most of the metallic lead originally present in the impure by-product lead sludge. A temperature of at least 700.degree. C. is required in the furnace to release molten metallic lead therefrom. The lower layer of molten lead is drawn off into a pump pot, a dross usually being formed during the pot filling and being recovered as skimmings, and returned to the alloying operation. The slag and the dross are then further treated to recover their lead content.
The slag material is composed mainly of lead oxide, occluded lead, sodium chloride and minor proportions of other metal compounds such as lead chloride, sodium hydroxide, sodium carbonate, sodium plumbite, and ash residue and, sometimes, minor proportions of sodium sulfide, sodium dichromate, lead sulfide and iron salts. Such slag material is normally in the form of a high melting, highly viscous, pasty or solid mass which holds a material amount of metallic lead dispersed therein, the metal compounds therein preventing the dispersed molten lead particles from coalescing and passing into the lower layer of molten lead. When the impure by-product lead steam still sludge contains iron or iron compounds, they also appear in the reverberatory furnace slag, tending to concentrate as solid particles at the interface of the slag and the molten lead layers, interfering with the efficiency of contact between those layers and the passage of the molten lead from the slag layer into the molten lead layer. Heretofore, such slag materials have been shipped to another plant for reclamation of their lead content, usually by treatment with iron oxide, calcium carbonate and coke at high temperatures in a blast furnace. However, other methods for lead recovery at various stages are known.
Denison, in U.S. Pat. No. 2,692,197, teaches a method of refining the impure by-product lead sludge by treating it with molten sodium hydroxide to form a lower layer of molten metallic lead and an upper layer of molten caustic containing the slag materials. The resulting mixture of caustic and slag materials must be further treated to recover the lead therefrom.
Larson, in U.S. Pat. No. 2,691,575, teaches the treatment of lead oxide slag materials with fused sodium hydroxide and metallic sodium to reduce the lead oxide to metallic lead. While effective, such a process, and the process of Denison, supra, have the disadvantage that they cannot be operated practically in the existing conventional reverberatory furnaces because the fused caustic, particularly in the large amounts employed, severely and rapidly attacks the ceramic linings of the furnaces. The replacing of such linings with suitable alkali resistant material is costly. Also, such processes require the use of considerable amounts of sodium hydroxide which are difficult to recover. Furthermore, in the process of Larson, it is difficult to convert all of the lead compounds to metallic lead.
Kreimeier in U.S. Pat. No. 2,899,296 teaches the treatment of lead by-product materials, such as reverberatory furnace slag, at a temperature of 400.degree.-850.degree. C. in the presence of a compound selected from sodium cyanide, potassium cyanide, mixtures of sodium cyanide and potassium cyanide, and mixtures of any of those with a hydroxide or the corresponding alkali metal, such that the alkali metal cyanide is present in proportion of about 20% excess over that theoretically required to react with the lead compounds and convert them to metallic lead. Although this process appears to be simpler in that only one step is required, it nevertheless suffers from the disadvantage of introducing cyanide compounds into the slag removed from the furnace and, thus, presents an additional waste disposal problem.
Padgitt in U.S. Pat. No. 2,765,328 teaches that the presence of a small amount of unreacted sodium metal from the tetraalkyllead reaction during smelting of by-product lead sludge in the lead recovery reverberatory furnace assures that the lead is collected as a homogeneous liquid phase virtually free of metal chloride originally present. The unreacted sodium metal comes from the sodium lead alloy used in the alkylation reaction. Thus, a lower yield provides unreacted sodium metal which is used to attain the advantageous smelting process result of lowering the sodium chloride content in the smelted lead and decreasing the smelting time. Although this process provides extremely efficient smelting operations, the lowered yield in production of tetraalkylled compounds is undesirable.
Another waste product produced in the manufacture of tetraalkyllead is an alkali metal in admixture with an alkaline earth metal, specifically, sodium metal mixed with calcium metal and their salts. Sodium is used as described above in an alloy with lead in the manufacture of tetraalkyllead. Sodium for alloying with lead is produced in one method by an electrolytic process, utilizing as the electrolyte a fused metal salt mixture. For example, in the production of sodium a fused mixture of sodium chloride and calcium chloride is employed as the electrolyte. The purpose of adding the calcium chloride is to reduce the melting point of the electrolyte to a temperature below the boiling point of sodium. Optionally, the fused salt bath can also contain barium chloride. In this way, the sodium produced at the cathode will be in the molten state and will not be vaporized at the temperature of the electrolytic mixture. While the use of the fluxing agent such as calcium chloride has this advantage, it does introduce additional difficulties into the overall process. The main difficulty is that calcium metal is produced at the cathode along with the sodium. This crude product is collected, cooled and subsequently filtered so as to recover most of the sodium in substantially pure form for use in alloying with lead. The residue or filter sludge which remains after this purification contains appreciable quantities of both sodium and calcium and it is the production of this sludge as a by-product which has presented a disposal problem to the industry for a number of years.
This sludge contains on the average about 90-95 weight percent sodium and calcium, the remainder consisting of the various salts and oxides of these metals and other impurities. The sodium content averages about 70 percent by weight and the calcium content varies between 5 and 30 percent. The sludge is in the form of crystals of calcium metal and electrolyte embedded in a matrix of sodium.
It is necessary to either convert this sludge into a usable commercial product or dispose of the material in some suitable manner. Although workable processes have been described for converting the sludge into useful products (note, for example, U.S. Pat. No. 2,759,896 and Canadian Pat. No. 836,447), these processes are not used commercially because of the costs and hazards associated with handling and processing the large volumes of sodium filter sludge produced in commercial facilities.
Accordingly, the disposal of such sludge is still the only practical and feasible alternative. One such disposal method presently in use is to collect the active sodium/calcium filter sludge in 55-gallon drums, transport the drums to a suitable ocean dumping site, perforate the drums and toss them overboard. The contact between the active metal in the sludge and the water is vigorous, but harmlessly converts the filter sludge to oxidation products of no real danger. This method is costly and involves transporting the sludge over considerable distances. A significant contribution to the art would be a safe, convenient, economical, controllable, complete and environmentally acceptable process or method for disposing of this sludge. Such a process or method is provided as one aspect of this invention.
Thus, this invention provides for recovery of by-product lead from reverberatory furnace lead slag and the use of waste sodium/calcium filter sludge as the major reductant in the process. Accordingly, the present invention provides many advantageous aspects which are illustratively described hereinbelow.